The present invention relates to a proportioning valve.
FIGS. 7 and 8 show a conventional proportioning valve.
In this proportioning valve 20, a fluid supplied from a master cylinder, etc. to an input pressure chamber 21 through an input port 21a is supplied to an output pressure chamber 24 through a space between a valve element 22a of a plunger 22 and a valve seat 23 as shown by solid-line arrows in FIG. 8, and then sent under pressure to a wheel cylinder, etc. through an output port 24a. When the pressure in the output pressure chamber 24 reaches a predetermined value, the differential pressure acting on the plunger 22 operates the plunger 22 against the energizing force of a spring 25, so that the valve element 22a of the plunger 22 abuts against the valve seat 23, thereby the output pressure chamber 24 being isolated from the input pressure chamber 21 (point A on the characteristic curve in FIG. 9). Then, when the pressure in the input pressure chamber 21 is increased, the valve element 22a of the plunger 22 separates again from the valve seat 23, so that the fluid in the input pressure chamber 21 is sent under pressure into the output pressure chamber 24. When the pressure in the output pressure chamber 24 reaches a predetermined value with respect to the pressure in the input pressure chamber 21, the valve element 22a of the plunger 22 abuts against the valve seat 23, by which the output pressure chamber 24 is isolated from the input pressure chamber 21. By repeating this operation, the output pressure is increased (characteristic line ACB in FIG 9).
This proportioning valve is provided with a check valve mechanism in addition to the above-described stop valve mechanism. This check valve mechanism is formed of a lip 23a of the valve seat 23. When the pressure in the output pressure chamber 24 becomes higher than the pressure in the input pressure chamber 21, the check valve mechanism returns the fluid in the output chamber 24 to the input pressure chamber 21.
However, between points A and B on the characteristic curve, the pressure in the input pressure chamber 21 is always higher than the pressure in the output pressure chamber 24, so that a state in which the valve element 22a abuts against the valve seat 23 is kept. Therefore, when the depressing force of master cylinder is gradually released, that is, when the pressure in the input pressure chamber 21 is gradually decreased, the plunger 22 rises slightly by deforming the valve seat 23, by which the volume of the output pressure chamber 24 is slightly increased. As a result, the pressure in the output pressure chamber 24 decreases slightly.
With the proportioning valve of this type, when the brake is released gradually, the pressure in the output pressure chamber 24 cannot respond quickly to the decrease in pressure in the input pressure chamber 21 as described above. Therefore, hysteresis occurs as indicated by CDA on the characteristic curve in FIG. 9.
Such a phenomenon is not undesirable particularly when an anti-skid control brake is used.